This invention relates to demodulators, and more particularly, to a spread spectrum demodulator for demodulating a spread spectrum four phase PSK RF signal.
In digital data transmission systems it is frequently desirable to provide a system of modulation which will increase the immunity of the transmission system to intentional jamming and to provide a highly secure data encoding system which can only be demodulated by selected receiver systems. One method of accomplishing these two goals is to provide a digital communication system in which the bandwidth of the transmitter signal is spread over a substantially greater bandwidth than the bandwidth of the data signal. This is generally accomplished by mixing the digital data signal with a wide band pseudo-random sequence of pulses. The psuedo-random modulated data signal then phase modulates a reference carrier signal.
This modulation method generates a wide band RF signal which is commonly known as a spread spectrum signal. Spread spectrum signals are highly secure in that only receivers capable of generating a pseudo-random sequence of pulses identical to the transmitted sequence will be capable of demodulating the digital data signal.
Prior art spread spectrum demodulators have incorporated four phase to two phase correlator circuits which perform adequately but are structurally relatively simple. As a result of the structural simplicity of these prior art correlators, the maximum achievable processing gain of the correlator is limited. Utilization of a minimum number of mixers in prior art correlator circuits often results in a less than desirable degree of isolation between the input of the correlator and its output. An additional disadvantage of the relatively straightforward prior art correlator circuit designs is that they provide only a minimal resistance to jamming. In a hostile electromagnetic environment which includes active electronic countermeasures systems, extremely high resistance to jamming is an absolutely essential requirement for the successful operation of a communications system.
Apparatus related to the present invention and which are designed to demodulate a spread spectrum signal modulated by a pseudo-random code sequency have been described in two commonly assigned pending United States Patent Applications entitled, "Spread Spectrum Demodulator" (Ser. No. 611,366) and "Improved Spread Spectrum Demodulator" (Ser. No. 611,367) which were filed simultaneously on September 8, 1975. Each of these pending patent applications contains background and disclosure material which is relevant to the description of the present invention.